I think the worst scam I've heard of so far is the Jehovah's Witness/Mormon impostors. They know on your door and tell you that they want to give you a brochure and tell you about a new center of their faith opening up and ask if you're interested. A lot of older people open the door in my neighborhood, for one, they're all lonely elderly women, so they're grateful to talk to anybody during the day, and a lot of kids (unfortunately) open their doors without looking because they think it's another kid or that because it's an adult they have to.
The scam is similar to the UPS guy scam; but it's disgusting that they hide behind a Kingdom Hall brochure and pin or a Church of Latter Day Saints jacket<--these aren't hard to get, my friend's boyfriend went as a mormon for Halloween and his costume was VERY official and convincing.
When they come to my door, I can look out my kitchen window and say that I'm not interested. If they persist, I'll ask them to leave once more. If they continue to persist, I show them that I'm one digit away from calling the police and that it would be wise of them to get the hell off of my property, which usually gets the job done.
When I was about 12, a guy named PJ came up to my door and asked to speak to my dad. He said that my dad used to be his little league coach (which was true) and that he'd invited him over to catch up (which was not true). I said that I couldn't let him in and that he could come back at 5 when my dad was home. I lied and said that my mom was asleep (I was home alone) and not feeling good so nobody could come in. He insisted on waiting on the porch and I kept asking him to just leave and come back when my dad's truck pulled up. He kept looking at me through the window and pacing on the porch. I shut the blinds, locked every door and window to our house and called my dad. My dad told me that he never invited PJ to our house and that he in fact hadn't seen PJ since he was about 13 years old. He said for me to look and see if he was still there, and he was, and he was still trying to look into the window, and the look on his face made me really uncomfortable and scared. My dad said he was gonna leave work early so that he could get home to me, but that it would be some time because of the traffic. I freaked out, I knew it would be more than half an hour before my dad came and I didn't know what I would do for a half an hour while this creep was pacing my front porch. My uncle had just moved around the corner from us, so I called him and asked him to walk by my house and maybe get PJ to leave. About 10 minutes after that I heard scuffling and yelling outside my house. My uncle had walked by, seen PJ, asked him what he was doing on our porch and then asked him to leave and PJ pulled a sizeable pocket knife out on my uncle and threatened him. I called the police and when PJ saw me on the phone he panicked and ran away, but he dropped his knife in our yard. The police came and took the knife and when i said "his name was PJ" they knew exactly who I was talking about. They were looking for him for a similar incident weeks prior. He knocked on a kids door to ask if he could use the phone, and the kid let him in and PJ pulled the knife out on the little boy and told him not to scream, or call anybody, then cut their phone line and stole a bunch of little valuables and electronics out of their house, then rode off on the boy's bike. It turned out that PJ was extremely emotionally unstable and very dangerous. If I hadn't been smart enough to know what to do about him being on our porch, he could have done something terrible to me.
Unfortunately, a lot of kids DON'T know that it's not okay to let someone in when they're home alone. And it's sad that we live in a society where you need expensive home security and surveillance on your front door before you can open it. Or that some of us feel the need to answer our door with a loaded weapon or other means of protection.